The majority of modern compact fluorescent lamps and fluorescent lamp ballasts operate with a common design principle. The typical design converts the incoming AC line power to a high DC voltage and then in turn, converts the DC voltage into a high frequency, high voltage square wave to drive connected fluorescent lamps. A typical compact fluorescent lamp ballast is described in U.S. Pat. No. 7,202,614, which is hereby incorporated by reference in its entirety.
As a result of this high frequency, high voltage square wave used to drive the fluorescent lamps, the circuit tends to create significant electrical interference that can come back across the AC power line, as well as be radiated into the air. This interference can cause problems with other electrical apparatus in the area. To minimize the effect of this interference, a filter, typically comprising an inductor and capacitor such as is shown in FIG. 6, is commonly employed to shunt the interference to ground.
Illuminated switches are designed to generate a small light so as to be as visible in a dark area. The typical illumination uses a neon lamp as shown in FIG. 5. One of the disadvantages of this type of circuit is that the neon lamp has limited life span, and after a few years the neon may not be bright enough, or may even fail to illuminate.
To solve this problem an LED may be used in place of the neon lamp. A properly chosen LED and associated circuitry will last many times longer than a neon lamp. Both LEDs and neon lamps operate on a similar principle of leaking a small amount of current through the connected load when the light switch is in the off position. However, where neon lamps typically operate on an AC voltage in this type of application, LEDs operate on a DC voltage. That is, by their nature, LEDs illuminate when current flows in one direction and prevent current from flowing in the opposite direction.
A conventional LED circuit for this application is shown in FIG. 1. A variety of circuits will illuminate the LED and allow the associated load, whether it is a fluorescent lamp, ballast or incandescent bulb to operate properly. Due to the nature of the LED, care has to be taken to use a proper circuit design for the illuminated switch to avoid problems not present in neon bulb circuits.
For example, if the LED circuit shown in FIG. 1 is used for the illuminated switch, flickering of the connected fluorescent lamps may occur. This flickering happens because DC is used to illuminate the LED, and that current continues to flow into the electronic ballast power supply capacitor (C2). When there is sufficient charge on capacitor C2, the drive circuit engages and fires the connected lamp(s). The capacitor then discharges and the cycle repeats, continuously, causing a disadvantageous flickering of the lamp.
When an illuminated switch employing a neon lamp as an indicator is used in this application, the flickering lamp problem will not occur, because a neon lamp operates on AC. That is, the neon lamp permits current to flow in both directions. The alternating current used to illuminate the neon lamp leaks through the electronic ballast circuitry as does the DC current for the LED, however the majority of the alternating current used for the neon lamp leaks through the capacitor (C1) used for the interference filter, such that capacitor C2 is not charged enough to engage the drive circuit for the lamp.
This insufficient charging is because an alternating current will pass through a capacitor, while a direct current charges the capacitor. Thus, there is not enough current available to charge the power supply capacitor in the electronic ballast circuit because the majority of the current is shunted by the interference capacitor (C1) before it can enter the power supply circuit of the ballast.
Accordingly, a need exists for an improved circuit for an LED for use in an illuminated light switch. That circuit will avoid disadvantages of conventional designs, including minimizing or avoiding flickering associated with the use of LEDs in conventional illuminated switch circuits.
Throughout the drawings, like reference numerals will be understood to refer to like features and structures.